Oxygenates can be catalytically converted to olefin product using a molecular sieve catalyst. The oxygenates are converted by contacting catalytic sites that are located within porous regions of the catalyst. As the oxygenate is converted to product inside these porous regions, the product tends to leave the pores. However, over reaction also occurs, which leads to a hydrocarbon layer, typically referred to as a coke layer, being formed on the surface and in the pores of the catalyst. Once the pores of the catalyst are filled or covered by this coke layer, no oxygenate can enter the pores and contact the active catalyst site for additional conversion, so the catalyst becomes deactivated. The catalyst can be re-activated under these circumstances by removing the coke layer. Conventionally, this re-activation is done by burning off the coke layer under relatively high combustion conditions, referred to as regeneration. The result is that the once coked catalyst has been made active, or activated, for re-use.
Catalysts containing silicoaluminophosphate molecular sieves are typically used to convert the oxygenates to olefin product, because the product typically contains a high amount of prime olefins, which are typically considered the low carbon number olefins such as ethylene and propylene. Some of these catalysts in their activated form have been found to be particularly sensitive to moisture effects, as prolonged exposure of the active catalysts to moisture can also deactivate the catalyst.
U.S. Pat. No. 6,316,683 B1 (Janssen et al.) discloses a method of protecting catalytic activity of a silicoaluminophosphate (SAPO) molecular sieve by shielding the internal active sites of the molecular sieve from contact with moisture. The template itself can serve as the shield, or an anhydrous blanket can serve as a shield for an activated sieve that does not include template. It is desirable to shield the active sites, because activated SAPO molecular sieves will exhibit a loss of catalytic activity when exposed to moisture.
U.S. Pat. No. 4,764,269 (Edwards et al.) discloses a method of protecting SAPO-37 catalyst from deactivating as a result of contact with moisture. The catalyst is maintained under storage conditions such that the organic template component of the molecular sieve is retained in the SAPO-37 molecular sieve, until such time as the catalyst is placed into a catalytic cracking unit. When the catalyst is exposed to the FCC reaction conditions, wherein the reactor is operated at 400° to 600° C. and the regenerator operated at about 600° to 850° C., the organic template is removed from the molecular sieve pore structure, and the catalyst becomes activated for the cracking of hydrocarbons. According to this procedure, there is little if any contact with moisture.
Mees et al., “Improvement of the Hydrothermal Stability of SAPO-34,” Chem. Commun., 2003, (1), 44-45, first published as an advance article on the web Nov. 22, 2002, discloses a method of protecting SAPO-34 molecular sieve, based on a reversible reaction of NH3 with acid sites of the sieve. The method transforms a H+-SAPO-34 into an NH4+-SAPO-34 in reversible way. The NH4+-SAPO-34 is said to be able to withstand severe steaming for an extended period of time without loss of structural integrity and acidity.
Oxygenate conversion reaction systems often incorporate the use of a fluidized bed reactor and regenerator, and operation of such systems is largely not practical without the use of steam injection. Typically the steam is used as motive and cooling source for the catalyst used, and some fluidized bed systems provide for removal of accumulated water. For example, U.S. Pat. No. 6,290,916 B1 (Sechrist et al.) discloses a method for removing water from a regenerator using circulating catalyst as an adsorbent.
Since various molecular sieves are susceptible to damage due to contact with water, and such systems are often used in conjunction with fluidized bed reaction systems, it would be beneficial to operate such systems at low moisture levels. Alternatively, it would be beneficial to operate such systems in such a way that the presence of water would have low or no impact on catalyst activity. Thus, additional methods of operating oxygenate conversion systems to minimize catalyst deactivation due to moisture effects are desired.